A defence system intended to destroy incoming hostile targets may comprise a radar location system and an attack system such as missiles or guns. It is known that the range of the attach system may be limited or the effectiveness reduced with range. Consequently, it maybe desirable to deal with the incoming target only when that target is within a specific range sphere. Thus, the radar system used will have a pulse repetition interval determined by this range.
Hostile targets outside the proposed range sphere may also be detected and a problem arises when a first time around targets i.e. within the range sphere nd a second or subsequent time around target i.e. outside the range sphere differ in range by an amount such that the delay between the echo pulses received from the different targets is a multiple of the radar systems pulse repetition interval. In such a case, echo pulses from different targets arrive at the radar receiver simultaneously.
A known method of relieving the multiple time around target problem is to transmit pulse trains having different pulse repetition intervals. The delay between the different echo pulses received provides a means of distinguishing the targets. This method however becomes inaccurate when a high duty cycle is used particularly when the transmitted pulse length is significant compared to the pulse repetition interval. When this occurs, echo pulses received from the different pulse trains overlap and it may be impossible to distinguish which echo pulse has been received from which target.